from __future__ import print_function
import time
import sys
from math import pi

sys.setrecursionlimit(10002)
#INFILE = open('1.in')
INFILE=sys.stdin

from math import gcd

def exgcd(a,b):
    x0=1;y0=0
    x1=0;y1=1
    x=0;y=1
##    r=a%b
##    q=(a-r)/b
    q,r=divmod(a,b)
    while r:
        x=x0-q*x1;y=y0-q*y1
        x0=x1;y0=y1
        x1=x;y1=y
        a=b;b=r
        q,r=divmod(a,b)
    return b,x,y

def equation(a,b,c):
    d,x,y=exgcd(a,b)
    if c%d:
        return False
    k=c//d
    x*=k;y*=k
    return x,y

def modular_inverse(a,n):
    r=equation(a%n,n,1)
    if r:
        t=r[0]
        while t>n:
            t-=n
        while t<=0:
            t+=n
        return t
    else:
        return False

def solve(w,n,d,nx):
    res=0
    g1=gcd(n,d)
    n//=g1
    d//=g1
    n1=modular_inverse(d,n)
    for i in range(w//2):
        a,b=nx[i],nx[w-1-i]
        df=abs(a-b)
        if df%g1:
            # print(i,a,b,df)
            return 'IMPOSSIBLE'
        df//=g1
        res+=min(df*n1%n,(n-df)*n1%n)
    return res


def read_input(fi):
    read = lambda type: type(fi.readline().rstrip('\n').rstrip('\r'))
    readArray = lambda type: list(map(type, fi.readline().split()))
    readMatrix = lambda type, x: [list(map(type, fi.readline().split())) for i in range(x)]
    readLines = lambda type, x: [type(fi.readline().rstrip('\n').rstrip('\r')) for i in range(x)]
    w,n,d=readArray(int)
    nx=readArray(int)
    return (w,n,d,nx)


def main():
    time0 = time.time()
    t = int(INFILE.readline())
    # _ = int(INFILE.readline())
    for ti in range(t):
        time1 = time.time()
        ans = "Case #%d: %s" % (ti + 1, solve(*read_input(INFILE)))
        sys.stderr.write("%.3f\t" % (time.time() - time1))
        print(ans)
    sys.stderr.write("%.3f\n" % (time.time() - time0))
    if INFILE != sys.stdin:
        INFILE.close()


if __name__ == '__main__':
    main()
    # print (check('RSP'*20))
    # print(check('R'*20+'S'*20+'P'*20))
    # print(check('R'*20+'S'*30+'P'*10))
    # print(test(16))
    # print(test1())